DSL (Digital Subscriber Line) is a widespread technology for data communication over existing telecommunications transmission lines.
ADSL2+, described in ITU-T standard G.992.5 and G.992.3 and VDSL2, described in ITU-standard G.993.2 are examples of such technology. ITU-T standard G.997.1 describes physical layer operation and maintenance functions for several DSL standards.
DSL communications equipment is often adapted to perform various operation and maintenance functions, for example to aid in diagnosing communication problems
ITU-T standards G993.2 and G997.1 describe inter alia that VDSL2 equipment can carry out measurements of quiet line noise (QLN) in the equipment's receive bands. This noise affects the signal to noise ratio (SNR) and hence the achievable data transmission rate and therefore there is a desire to measure this parameter, e.g. for diagnostic purposes. If the transfer function, H, of the line is also measured, the transmit power needed to achieve a particular data rate may be calculated.
In general, there is a desire to provide improved transmission line diagnostics, for example to provide improved diagnostics of noise problems.